For two or more syllabled verbs ending in a single vowel plus a single consonant, you should add more explanation and especially the stress of the verbs should be explained clearly. For example: open -opened, visit-visited, answer-answered.... because these verbs are stressed at the beginning, but for these verbs below which are stressed at the end , so they need to doubled a final consonant before adding-ed. Ex: admit-admitted, refer-referred, commit-committed....etc.